Lpk's, bolts, ect?

Also is the buffer tube same diamter and threat as a milspec 556 tube?

I don't have it handy but ArmaLite put out a tech note.

Uppers are different due to mag dimensions but only the receiver is different, not the small parts.
Bolt catch and mag catche are model/receiver-specific.
There is probably a couple more lower parts I'm forgetting.

Allegedly a B upper can be machined so that it will allow mag insertion and feeding on an A.
The downside is that you get shiny un-anodized aluminum unless you cough up for proper re-anodize.